In the design of integrated circuits (ICs), such as memory chips, off chip drivers (OCDs) are employed to transfer data information from the memory chip to the external environment. The OCD generally comprises a plurality of transistors, including n-type field effect transistors (n-FETs) and p-type FETs (p-FETs) configured to drive a chip's internal data signal as fast as possible to the outside system environment, which represents a heavy load (.apprxeq.100 pF).
To improve noise characteristics, such as reducing ringing caused by leadframe inductance, the OCD is usually provided with multiple stages. For example, in a two stage OCD, the output of the second stage can be delayed by a predetermined delay with respect to the output at the first stage.
Continuous demand for the miniaturization of devices has resulted in smaller and smaller feature sizes. For example, the gate lengths of current n-FETs are about 0.25 microns (.mu.m). Such lengths are, however, susceptible to hot carrier degradation. This occurs in the presence of high drain-source voltages during switching of the gate (worst case condition: gate voltage=1/2 drain-source voltage).
To reduce or prevent hot carrier degradation as a result of high source-drain voltages, a first n-FET transistor is stacked or placed in series with a second n-FET. The gate of the first or stacked n-FET is coupled to V.sub.DD, thus providing a voltage drop of V.sub.t (where V.sub.t is the gate threshold of the stack n-FET). Due to the body effect, this voltage drop is actually even greater. The voltage drop provided by the stack n-FET lowers the source-drain voltage of the second n-FET sufficiently during switching to reduce or prevent hot carrier degradation.
Although the stacking configuration effectively reduces hot carrier degradation, it however results in relatively high output capacitance. This is because stacking of the transistors doubles effective gate length, which for performance reasons requires a widening of the transistors. Such increase in output capacitance can exceed specified limits. For example, a conventional OCD without the stacked configuration has a typical output capacitance of about 4.5-5 pF. When incorporating a stacked configuration, the total capacitance approaches or exceeds the maximum allowed specified value, which is about 7 pF. This is because stacking effectively doubles the transistor length, which requires doubling of width to achieve the same performance.
Thus, it is desirable to provide an OCD with low output capacitance without the need to sacrifice performance.